(1) Field of the Invention
The present invention generally relates to a system for preventing dust build-up in ductwork. More particularly, the present invention relates to a system that uses the injection of air to re-entrain or fluidize ash in flue gas flowing through the ductwork of a selective catalytic reduction (SCR) system.
(2) Description of the Related Art
Selective catalytic reduction (SCR) systems are commonly applied to utility and industrial combustion units to reduce NOx emissions. In an SCR system, ammonia or the like is injected into a flue gas. The flue gas injected with ammonia is passed through a catalyst where chemical reactions occur to convert NOx emissions to elemental nitrogen and water. The presence of a catalyst is generally required to accelerate the chemical reactions because SCR systems typically operate at relatively low temperatures, which may slow or prevent the chemical reactions. Commonly used catalysts include a vanadium/titanium formulation, zeolite materials, and the like.
Many of the installations place the SCR reactor in high dust locations before the particulate collection system. Careful attention is paid to the design of the ductwork and SCR reactor to avoid dust deposition. The catalyst is designed specifically to withstand the erosion and potentially poisonous effects of the fly ash. The ductwork velocities are chosen to ensure the fly ash remains entrained at the design point, because ash drop out in the ductwork is undesirable.
However, it is common for such systems to experience dust deposition in some locations within the ductwork under certain circumstances. The reduction in gas velocity through the ductwork experienced when the combustion unit is operated at reduced loads is the main cause of dust deposition. It could also be caused by environmental changes in the operating of the unit, for example, operating with lower excess air, or different fuels. The most common points for deposition are dead legs in the ductwork and in the ductwork just upstream of the SCR inlet hood.
FIGS. 1 and 2 provide an example of dust build-up and resulting plugging of a SCR system 20 from ash accumulation. FIG. 1 shows a portion of SCR system 20 when the combustion unit is operating at a low load 22. SCR system 20 is typically located between a steam generator outlet (not shown) and a pre-heater inlet (not shown). As a flue gas stream 21 flows through a duct 24, fly ash is typically present in the flue gas stream. A catalyst 26 is housed in SCR system 20 within duct 24 and is subjected to the full concentration of fly ash as the flue gas stream 21 passes through it. Catalyst 26 is typically covered by screens 28 to capture fly ash before it reaches the catalyst channels (not shown).
SCR system 20 is sized to receive flue gas stream 21 when the combustion unit (not shown) is operating at a full load. When the combustion unit (not shown) is operated at a low load 22, duct 24 has less flue gas passing through it. The velocity of flue gas stream 21 is therefore reduced greatly. This reduction in velocity can lead to dust deposition. As flue gas stream 21 flows through duct 24, a fly ash 30 accumulates and settles in a dust pile 32. Due to the design of duct 24, dust pile 32 normally occurs just upstream of an SCR inlet hood 34.
Referring now to FIG. 2, when SCR system 20 is operating at a full load 36, the velocity of flue gas stream 21 increases back to the design velocity. As the velocity is increased to accommodate full load 36, fly ash 30 that has accumulated in dust pile 32 may re-entrain suddenly causing an avalanche 38 of the fly ash to fall onto catalyst 26. As a result, channels (not shown) within catalyst 26 may become plugged and the efficiency of SCR system 20 reduced. The pressure drop across SCR system 20 may also increase.
Typically, the only measures taken to prevent the build-up of dust piles involve the design of the ductwork. Generally, the shape of the entrance to the SCR inlet hood can be designed such that the velocity through this transition piece is constant at the design point. The result is ductwork with a sloping roof that is at the same time, expanding to match the SCR reactor cross-section. Bypass ducts are protected either by equipping them with dampers to eliminate dead legs or by making the bypass duct have no shelf where ash can accumulate.
These approaches have generally been proven unsuccessful. The issue of dust deposition at the SCR inlet hood entrance and dead legs in the ductwork still remains. Ash piles being sloughed off onto the catalyst beds as the combustion unit comes back up to full output load is an issue. Current technology offers little to address the potential of ash deposition at the SCR reactor inlet area.